Modern Land Mobile Radio (LMR) communications systems typically have a number of geographically distributed receive-only sites associated with each receive-and-transmit site in the system. The need for these receive-only sites can be understood by considering a simple LMR system having a single receive-and-transmit site. The transmitter of this single receive-and-transmit site is designed to service a desired geographical area by an appropriate choice of the factors that contribute to its Effective Radiated Power (ERP). For instance, the receive-and-transmit site is typically located at a relatively high elevation, such as on the top of a hill, mountain or tall building, and has a relatively high-powered Radio Frequency (RF) transmitter so that all the LMR transceivers operating within the desired service area can revive the signals it transmits at an acceptable strength.
However, the mobile and portable RF transceivers served by the receive-and-transmit site typically have considerably smaller ERPs because of power, size and cost considerations. For instance, the receive-and-transmit site typically has access to grid or generator power and can produce RF transmission signals of several hundred watts, whereas a mobile transceiver operating on its own batteries, or via a vehicle's electrical system, is typically capable of generating an RF transmission signal of only 5 to 25 watts. A portable or hand-held transceiver, having a few small nickel-cadmium battery cells as its power source, is even more restricted and may generate an RF transmission signal of only 1 watt or less. The result is that while all the LMR mobile/portable transceivers within the service area can typically receive the strong transmissions from the receive-and-transmit site, the receive-and-transmit site may not receive the weaker transmissions from the mobile and portable transceivers or may receive the transmissions at signal strengths that are too low to provide useful, reliable communications. In other words, the “talk in” or “talkback” range of the receive-and-transmit site is typically significantly less than its “talk out” range.
Having multiple receive-only sites at various geographical locations throughout the LMR service area can solve this problem as long as at least one of the receive-only sites, or the receive-and-transmit site, receives any particular transmission at an acceptable signal strength level. Once a receiver somewhere within the LMR system receives one acceptable version of a transmitted signal, that version of the signal can be relayed back to the receive-and-transmit site by landline and re-transmitted by the main site at a sufficiently high power level that all transceivers in the service area are able to receive the relayed signal an acceptable signal strength level. However, depending upon where the transmitting transceiver is when it transmits the original signal, more than one the receive-only sites will typically receive the original transmission at an acceptable signal strength level. The LMR system therefore needs to select one version of the received transmission signals, preferably the best one, and to ignore the rest. In existing system this selection is accomplished by a process referred to as “voting, which requires specialized voting equipment (also referred to as the “voter” or “the selector”). Typically the voter is connected to each of the receiving sites by landlines. Sites that receive the transmission relay the signal to the voter via these landlines. The voter then analyzes each received signal in turn and determines which one is the best. This determination is based on, for instance, the signal-to-noise ratio of the signal or a bit error rate in a standard part of the signal, such as a frame header. Once the voter has selected the best signal, and determined which receiver sent it, the voter proceeds to relay signals from that receiver to the receive-and-transmit site by landline for transmission. Such voting systems have been described in detail in, for instance, U.S. Pat. No. 4,013,962 titled “Improved Receiver Selecting (Voting) System” issued to Beske et al. on Mar. 22, 1977 and U.S. Pat. No. 5,131,007 titled “Digital Voter for Multiple Site PSTR Trunking System” issued to Brown et al. on Jul. 14, 1992, the contents of both of which are hereby incorporated by reference.
The voting systems described above use special voting equipment. They also introduce significant delays (also known as latency) into the transmission due to buffering required to ensure that the transmitter does not under run, i.e. does not begin transmission before all the necessary data has been received, and to ensure that all receiving sites have sufficient time to get their data to the voter. Existing digital voting systems also require a synchronized data interface between the RF receiving sites and the voting equipment. This synchronization allows the voter to poll the receiving sites serially. Synchronization also allows the voter selected receiving site to transmit its received signal by landline to the voter in a specific time window. This received signal is temporarily stored, or buffered, at the voter before being relayed on to the receive-and-transmit site for retransmission.
The problems associated with this approach to digital voting include the significant investment in voting equipment, the need for synchronized data paths, the latency introduced into the transmission by the buffering during transfer of the synchronous data and the need for the voter to poll all receivers serially.